linux-kernel - Re: [PATCH V3 6/9] virtio-ccw: implement synchronize

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Date:   Thu, 28 Apr 2022 09:42:17 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Halil Pasic <pasic@...ux.ibm.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        Stefano Garzarella <sgarzare@...hat.com>,
        eperezma <eperezma@...hat.com>, Cindy Lu <lulu@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Marc Zyngier <maz@...nel.org>
Subject: Re: [PATCH V3 6/9] virtio-ccw: implement synchronize_cbs()

On Thu, Apr 28 2022, Jason Wang <jasowang@...hat.com> wrote:

> On Thu, Apr 28, 2022 at 1:55 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>>
>> On Thu, Apr 28, 2022 at 01:51:59PM +0800, Jason Wang wrote:
>> > On Thu, Apr 28, 2022 at 1:24 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>> > >
>> > > On Thu, Apr 28, 2022 at 11:04:41AM +0800, Jason Wang wrote:
>> > > > > But my guess is that rwlock + some testing for the legacy indicator case
>> > > > > just to double check if there is a heavy regression despite of our
>> > > > > expectations to see none should do the trick.
>> > > >
>> > > > I suggest this, rwlock (for not airq) seems better than spinlock, but
>> > > > at worst case it will cause cache line bouncing. But I wonder if it's
>> > > > noticeable (anyhow it has been used for airq).
>> > > >
>> > > > Thanks
>> > >
>> > > Which existing rwlock does airq use right now? Can we take it to sync?
>> >
>> > It's the rwlock in airq_info, it has already been used in this patch.
>> >
>> >                 write_lock(&info->lock);
>> >                 write_unlock(&info->lock);
>> >
>> > But the problem is, it looks to me there could be a case that airq is
>> > not used, (virtio_ccw_int_hander()). That's why the patch use a
>> > spinlock, it could be optimized with using a rwlock as well.
>> >
>> > Thanks
>>
>> Ah, right. So let's take that on the legacy path too and Halil promises
>> to test to make sure performance isn't impacted too badly?
>
> I think what you meant is using a dedicated rwlock instead of trying
> to reuse one of the airq_info locks.
>
> If this is true, it should be fine.

FWIW, that approach makes sense to me as well.